1. Field of the Invention
The present invention relates to a pneumatic tire.
2. Description of the Related Art
In a pneumatic tire, specifically, a heavy-duty pneumatic tire mounted on a steering shaft, uneven wear occurs in a rib portion on a tread on a tire width direction outer side (in a shoulder area). The uneven wear occurring in the rib portion includes step wear, which results in a step extending from a circumferential main groove, due to toe-in, the camber, taking a sharp curve, or obliqueness of the road surface. Such an uneven wear occurring in the rib portion may interfere with vehicle handling.
In a conventional pneumatic tire, main grooves and ribs linearly extending in the tire circumferential direction are provided, and a shoulder rib on a tread on the outermost side in the tire width direction is not smaller than 1.35 and not larger than 1.6 of those of other ribs. In the conventional pneumatic tire, the linear main grooves and the linear ribs reduce the amount of uneven wear in each rib. In addition, by adjusting the ratio between the widths of the shoulder rib and other ribs in consideration of the rigidity of the shoulder rib, uneven wear is prevented from occurring in the shoulder rib. In the conventional pneumatic tire, to improve the wet braking performance even when the main grooves and the ribs linearly extending in the tire circumferential direction are provided, shallow narrow grooves extending in the tire width direction and sipes positioned on the groove bottoms of the narrow grooves are provided in each rib, and a total width of the sipes in the tire width direction each having a depth from the surface of the rib larger than 80% of that of the main groove is set to a width not smaller than 0.2 of the total width of the rib (see, for example, Japanese Patent Application Laid-open No. 2000-168317).
In the conventional pneumatic tire, while the ratio between the widths of the shoulder rib and the widths of other ribs is adjusted to optimize the rigidity of the shoulder rib and improve the uneven wear resistance, no consideration is paid for a belt layer in the tread. In a shoulder area, depending on the position of tire width direction outer ends (ends on the tire width direction outer sides) of the belt layer, the rigidity of the shoulder rib changes, which changes the uneven wear resistance. Therefore, in the pneumatic tire in which no consideration is paid for the belt layer, while the wet braking performance improves, the uneven wear resistance does not sufficiently improve.